Electrophotographic product and method for achieving electrophotographic copying



Aug. 27, 1968 J. CLANCY 3,399,060

EL ROPHOTOG HIC PRODUCT AND METHOD FOR HIEVING PHIG G ELECT PHOTOGRACOPYIN iled A l 16, 1963 PHOTOCONDUgTIVE WIRE .L, H FIL r34.

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INVENTOR.

John J. Clancy Attorney United States Patent 3,399,060ELECTROPHOTOGRAPHIC PRODUCT AND METH- OD FOR ACHIEVINGELECTROPHOTOGRAPHIC COPYING John J. Clancy, Westwood, Mass., assignor toArthur D. Little, Inc., Cambridge, Mass., a corporation of MassachusettsFiled Apr. 16, 1963, Ser. No. 273,404 12 Claims. (Cl. 96-1.4)

ABSTRACT OF THE DISCLOSURE A printing base and a method forelectrostatic reproduction. A photoconductive layer in the form of anessentially continuous film having voids ranging between 0.5 and micronsis carried on a substrate. The film may be formed of an organic materialwhich is itself photoconductive, or it may be a nonphotoconductivematerial which contains photoconductive particles. The photoconductivelayer may be made relatively light in weight. It is also, on an equalbasis, more sensitive than a film formed without voids.

Electrostatic printing is a process for producing a visible record, areproduction or a copy which includes as an intermediate step theconversion of a light image or image signals to an electrostatic chargepattern or image on a printing base. In the case of electrophotographicprocesses an electrostatic latent image or an electron latent image isproduced on a charged surface by utilizing the property ofphotoconductivity (i.e., a variable conductivity depending upon theintensity of illumination which strikes the coating). As will becomeapparent in the following description the electrostatic latent image maybe produced in a conventional exposure operation such as, for example,through a negative, or it may be created through the use of mirrors orother optical devices. The latent image thus created may be developeddirectly by toning with a charged powder, liquid or aerosol; or theimage may be toned and the toned image transferred to a second substratewhere the image is fixed.

In electrophotographic processes the electrostatic latent image iscommonly formed on the surface of a photoconductive dielectric layercarried on a supporting substrate. Typically, materials comprising asubstrate and a photoconductive layer thereon are sensitized by applyinga uniform surface charge to the free surface of the photoconductivelayer. Such surface charge may be applied for example by means of acorona discharge, which charge is retained owing to the substantialinsulating character, that is, the low conductivity of the layer whennot exposed to light. On exposure to light, the photoconductive propertyof the layer causes the conductivity to increase in the illuminatedareas, the extent of conductivity being dependent upon the intensity ofthe illumination. The surface charge in the illuminated areas leaks awayleaving the charge located in the unilluminated or unexposed areas whereit is desired to produce indicia. This remaining charge constitutes thecharged pattern or electrostatic latent image, which then may bedeveloped and used directly or transferred to a second substrate.

Such electrophotographic reproduction processes have found wideacceptance in recent years, particularly in the field of ofiice copying.They possess many inherent advantages, among which may be listed theelimination of liquid developing agents, the elimination of anyintermediate step using a photosensitive plate if the originally chargedmaterials are developed directly, and the ability to achieve continuoustone images.

Normally in the process of electrostatic printing a nonice conductivesubstrate such as paper, or a conductive substrate such as aluminumfoil, is coated with a material which contains a finely dividedphotosensitive material. Such photosensitive or photoconductive materialis normally zinc oxide, although other materials such as lead iodide,arsenic trisulfide, or cadmium selenide may be used. These particlesbeing photosensitive are capable of electrical conduction upon exposureto light. As an alternative to using photoconductive particles suspendedin a binder material, it has also been found possible to make aphotoconductive film of certain film-forming materials. Typically, sucha film-forming material is poly(N-vinyl carbazole). However, a number ofother organic photoconductors have been disclosed. (See for example US.Patent 3,041,165.)

In the use of the photoconductive particles suspended in a binder, it isfrequently desirable to restrict the choice of binders to certainspecified materials, and to add to the photoconductive coating a dyecapable of enlarging the absorption band of the coating material.Moreover, in order to obtain a satisfactory coating which can be used inthe manner described it has always been necessary to use relativelyheavy coating weights, i.e., of the order of about 30 pounds of coatingper ream (3000 square feet) of paper or other substrate. Consideringthat paper used for the purpose is typically 51 pound paper (3000 squarefeet ream basis), it will be seen that the finished photoconductivecoated paper weighs 81 pounds-a relatively heavy paper by modernstandards. Thus, if it were possible to make an equally efficientphotoconductive coating which was considerably lighter than that nowused, the resulting savings in weight handling alone would be material.

We have found that much lighter weight photoconductive coatings can bemade by forming the binder materials into films which contain a largeproportion of air voids. Not only does this film structure reduce weightbut, unexpectedly, it provides a markedly more eflicient photoconductivematerial, being more sensitive to a fixed light flux and hence a fasterfilm when used in the electrostatic reproduction process describedabove.

It is, therefore, a primary object of this invention to provide a noveltype of coating suitable for application to a substrate, the finishedcoated substrate to be used in an electrophotographic or electrostaticprinting process. It is another object of this invention to provide acoating of the character described which is materially lighter in weightand more sensitive to light than the presently available coatings. It isyet another object of this invention to provide such a coating which maybe formed using many binders which are not usable in the present type ofphotosensitive compositions. It is yet another object of this inventionto provide photosensitive compositions, suitable for coating on asubstrate, which are capable of exhibiting improved water resistance,and of being applied as a papermaking step, thus saving an additionalconverting step. Another primary object of this invention is to providean improved coated substrate suitable for use in electrostatic copyingmethods. Another object is to provide a coated substrate of thecharacter described which is lighter in weight and which is moresensitive to light and less sensitive to moisture than those now in use.Other objectives of the invention will, in part, be obvious and will, inpart, be apparent hereinafter.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others and thearticle possessing the features, properties, and the relation ofelements which are exemplified in the following detailed disclosure, andthe scope of the invention will be indicated in the claims.

The photoconductive coating of this invention may be described as abinder in the form of an essentially continuous film containingmultitudinous air voids uniformly distributed throughout, the air voidsvarying in maximum dimension between about 0.5 micron and 10 microns.The film, without photoconductive particulate matter, may be furthercharacterized as having a density of between about 0.1 gm./cc. and 0.6gm./cc. depending at least partly on the binder used. If the binder is adielectric material which is not itself photoconductive in nature, thefilm will also contain finely divided photoconductive particlesuniformly distributed throughout the essentially continuous binder filmmatrix. The structure of the finished coating of this invention may befurther characterized as having a vastly increased surface area byreason of the voids. Because of its structure, a coating made inaccordance with the teaching of this invention in a thickness equivalentto the thickness of prior art solid film coatings will weigh only about15% to 30% that of the solid film coating. Applying this to a practicalexample, it will be seen that instead of converting 51- pound per reampaper (3000 square feet) to 81-pound per ream paper to make it aphotosensitive sheet the coating of this invention converts the same51-pound per ream paper to 60 to 63 pound paper to achieve the same orimproved reproduction. Because lighter weight coatings are used, thisinvention anticipates the use of a lighter weight substrate, thus givingan appreciable reduction in the total weight of the product.

The voids in the coating may be obtained by several methods as willbecome apparent from the description below. They may, however, begenerally described as having been created through the formation ofpartially open air-binder interfaces, or through the formation ofessentially enclosed voids. 'Ihe film formed with partially openair-binder interfaces exhibits a high order of opaqueness and brightnessdue to the structure of the film which achieves good light scattering.

The voids in the finished photosensitive coating must vary within aspecified size range as defined below in order to be most effective, tobalance the optical and electrical properties of the finished coating,as well as to achieve acceptable strength in the coating itself.

In the coating of a substrate to form the photosensitive material ofthis invention it is preferable to apply the original liquid coatingcomposition in the form of an emulsion, the discontinuous and continuousphases being formed of immiscible liquids which exhibit a difference invapor pressure at the temperature at which drying of the coating takesplace. Coatings which are applied as emulsion films are dried in amanner to retain the original emulsion structure in the dried film, thusthe emulsion is not broken and the film-forming binder of the coating isnever converted to finely divided particles, but remains in the form ofa continuous film containing the necessary voids uniformly distributedthroughout.

Suitable coating emulsions may be the oil-in-water type or thewater-in-oil type, depending upon whether the film binder material issoluble in water or oil. If the film binder material is soluble inwater, then the emulsion will be of the oil-in-water type; while, on theother hand, if the film binder material is water insoluble, it will beof the water-in-oil type. If the finely divided particulate matter, suchas zinc oxide, is used in the coating to give it its photosensitiveproperties, then this finely divided photoconductive material ispreferably introduced into that phase of the emulsion which becomes thecontinuous film of the finished coating, that is, it will be introducedinto the continuous phase whether it is the oil-soluble or water-solublephase. Inasmuch as organic polymeric binder materials which inthemselves are photosensitive are not normally water soluble, they willbe made into water-in-oil emulsions, the binder material being dissolvedin a suitable organic water-immiscible liquid.

Techniques other than the use of emulsion coatings for introducing therequired voids into the finished photo- Cit conductive films are notprecluded. Thus, the film coating may be deposited as a foam, or as aliquid containing a volatile blowing agent. The primary requirement forany void-containing film forming technique is that the voids formed arewithin the size range specified.

For a fuller understanding of the nature and objects of the inventionreference should be had to the following detailed description taken inconnection with the accompanying drawings in which:

FIGS. 1 and 2 are fragmentary cross-sectional views of a substratecarrying the photoconductive coating of this invention containing afinely divided photoconductive material, such as Zinc oxide;

FIG. 3 is a fragmentary cross-sectional view of a substrate carrying aphotoconductive film of this invention made from a photoconductiveorganic film-forming material;

FIG. 4 is a cross-sectional view showing the manner in which aphotoconducting paper is charged;

FIG. 5 illustrates the step of exposing a charged photosensitive paperto a master negative to form a latent electrostatic image;

FIG. 6 illustrates the step of toning the exposed paper of FIG. 4; and

FIG. 7 illustrates the transfer of a toned image to a second substrate.

The structure of the photoconductive film of this invention is shown indiagrammatic fashion (much enlarged and not to scale) in FIGS. 1-3.FIGS. 1 and 2 show a coating which is made photoconductive by virtue ofthe presence of finely divided particulate matter within the continuousbinder film, zinc oxide being used for illustration only. In FIG. 1 thesnbstate 10, which may be an electrically nonconductive material such aspaper or an electrically conducting material such as a metal foil,carries on its surface the coating 11 which in this case is seen to beformed of a continuous binder material 12 having distributed uniformlythroughout it finely divided particulate matter 13 which in this exampleis photoconductive zinc oxide. Also distributed throughout thecontinuous film binder matrix 12 are a multiplicity of air binderinterfaces 14. FIG. 2 illustrates an alternative coating which is formedin such a way that the voids 14a are essentially enclosed rather than inthe form of the partially broken air-binder interfaces 14 of FIG. 1. Ingeneral, emulsion coatings of the oil-in-water type will form films likethat illustrated in FIG. 1; while emulsion coatings of the Water-in-oiltype will form films like that illustrated in FIG. 2.

FIG. 3 illustrates another embodiment of the photoconductive material ofthis invention. This film is formed of an organic film-forming materialwhich in itself is photoconductive. In this case the coating 11 is seento consist of the continuous film 15 having distributed throughout theair-matrix interfaces 14 throughout its entire volume. These air-matrixinterfaces make up the necessary air voids in the finished coating 11.

FIG. 4 illustrates the electrical charging of the photoconductivematerials of FIGS. 1-3 for further processing. Conveniently this is doneby locating the coated substrate on a grounded metal conductor 18 andplacing above it a series of fine wires 19 (only one of which is shownin the cross-section of FIG. 4). When a high DC voltage of the order of5000-6000 volts is applied across the wire and the grounded metalconductor, the coating 11 is charged on its surface. It is then ready tobe exposed to a photographic negative or to other master copy in such away that the light striking the charged coating will vary over thesurface corresponding to the intensity of the image to be reproduced. Asillustrated in FIG. 5, light from a source 23 is directed through amaster copy 24 which is to be reproduced. In this case the letter Adesignated by numeral 26 is the indicia which is to be duplicated on thephotosensitive paper 16, positioned below the master 24. It will beappreciated that considerably more light passes through the lighter areaaround the letter A than through the letter A. Thus, there is created onthe photosensitive paper 16 an indicia corresponding to letter A in theform of an electrostatic image thereon. Where light has been permittedto pass through and strike the photosensitive paper 16 the charge isdissipated and leaks away. However, where it does not strike thephotosensitive paper 16 (i.e., in that area defining the letter A) thereremains an electrostatic charge on the surface of the paper. Thiselectrostatic image may then be stored for a time but normally it willbe developed to render it visihle or to render it in a form which can betransferred to a second substrate. Development normally takes the formof applying a toner to the surface 27; the toner being a material whichbears the opposite electrostatic charge from that of the electrostaticimage. Hence, it is attracted and held to the image such as shown inFIG. 6. With the application of a toning material 29 from a source 30there is formed upon the sheet 16 a visible image 28. The toner materialmay be in the form of a powder, a liquid or an aerosol in keeping withwell-developed techniques. New toning techniques are not a part of thisinvention and any toner which is usable in this process may be used inthe method of this invention.

Once the visible image has been established on the sheet 16 it may befixed to it such as by heating or by the use of a solvent which willcause the toning material to permanently adhere to the substrate.Alternatively, the toned, but unfixed, image as illustrated in FIG. 6may be used to transfer to another substrate where it is fixed aspermanent copy. The transferring of the toned image to a secondsubstrate is normally carried out by using an electrically conductingplate of opposite charge to the toner used and placing it under thesubstrate to which the toned image is to be transferred. The firstsubstrate may then be used again. This step is diagrammaticallyillustrated in FIG. 7. A second substrate 32 is placed in contact withthe first substrate 16 bearing the toned and electrically charged image28. (It will be appreciated that virtual contact between the substrates16 and 32 is required and that the separation between them in FIG. 7 ismade for purposes of ease of illustration only.) An electrically chargedplate 33 deriving a charge from a source 34 is placed in contact withthe surface of the substrate 32 which is not in contact with the tonedimage. The charge on the plate 33 must, of course, be opposite inpolarity to the toned image to attract the toner to substrate 32 whichmay be of any material suitable to permanently affix the toned imagethereto.

In addition to the procedures which produce a latent electrostatic imageand then develop this image as illustrated in FIGS. 4-7, it is possibleto form a latent electron image, or latent conductivity image on thephotoconductive film. This process differs from that described above inthat a dark adapted photoconductive layer is first exposed to light asshown in FIG. 5, thus exciting electrons from the valence band intoshallow traps lying near the conduction band. This latent image isdeveloped by sprinkling toner over the surface and then applying apotential between the toner and the ground plate on which thephotoconductive sheet lies. When the toner is positive and the groundplate negative excited electrons are extracted from the portion of thelayer that was previously exposed to light, and these electronsneutralize the toner which subsequently falls off. The areas that werenot previously exposed to light are incapable of neutralizing the toner,which is, therefore, electrostatically bound to the photoconductivesheet. The image can then either be transferred to another sheet orfixed as described above.

It will be seen from the above description of the process ofelectrostatic printing that the coating must possess certaincharacteristics in order to make it an effective product. It must, ofcourse, be photosensitive and it must also be capable of holding .acharge deposited on its surface. Further, it is highly desirable thatsuch a coating be moistare-insensitive to make it operable underessentially all conditions of humidity without any marked variations inits performance. Such a coating should also be insensitive to pressureto be encountered in its normal handling. Moreover, it should respond toa relatively wide spectral band. If the coated substrate is to be usedas the final copy, then it should also be a coating which in itselfoffers a pleasing background as measured in terms of opaqueness andbrightness. Finally, it should be lightweight and inexpensive.

Many of the photosensitive particulate materials, e.g., zinc oxide,which are suitable for photoconductive papers and the like are,unfortunately, sensitive to a very narrow wavelength band in thespectrum normally used in exposing them. In keeping with well developedtechniques, this invention contemplates the addition of certain dyes tothe photoconductive coating to widen the spectral band to which it maybe responsive, if such dyes are needed. Each photoconductive materialhas a peak response characteristic of its particular composition and isinfluenced by such ingredients as the binder used in the coating. Forexample the peak response of zinc oxide is 3750 A., of thallium iodideabout 4130 A. and of silver sulfide about 13,500 A. The dyes which maybe added will of course be chosen to complement and broaden the spectralresponse which includes the peak response region of the photoconductivematerial. For example, xanthane, thiazole, thiazine, and diphenylmethanedyes have been found to be particularly Well suited dyes for use withzinc oxide, and conventional binders.

Typical dyes which are suitable for incorporation into photoconductivecoatings to broaden their spectral response include the xanthane dyessuch as Uranine (CI Acid Yellow 73), Eosine (CI Acid Red 87) and RoseBengal (CI Acid Red 74); the triarylmethane dyes as Crystal Violet (CIBasic Violet 3), Brilliant Green (CI Basic Green 1) and Patent Blue (CIAcid Blue 9); the thiazol dyes such as Thiofiavine TG (CI Basic Yellow1); the thiazine dyes such as Methylene Green (CI Basic Green .5) andMethylene Blue (CI Basic Blue 9); the azine dyes such as MethyleneViolet (CI Basic Violet 5); the acridine dyes such as Acridine Orange(CI Basic Orange 14); the diphenylmethane dyes such as Auramine 0 (CIBasic Yellow 2); the cyanine dyes such as Thiazole Purple(3,3'-diethylthiacarbocyanine iodide); anthraquinone dyes such asAlizarine Red (CI Mordant Red 3); and mixtures of dyes such as MethyleneGrey (CI Basic Black 1).

The following examples which are meant to be illustrative and notlimiting are given to further describe the method and product of thisinvention.

EXAMPLE 1 An oil-in-water emulsion was made using casein dispersed inwater as a continuous phase and a water immiscible solvent such askerosene as the discontinuous phase. 415 grams of a 12% caseindispersion in water was mixed with 300 grams of zinc oxide(photoconductive grade) and 50 grams of water. The zinc oxide used wasmade by the French process and is sold as Florence Green-Seal zinc oxideby New Jersey Zinc Company.

The ratio of zinc oxide to casein in this mixture was 6 to 1. To 200grams of the casein-zinc oxide mixture was added 3 grams of ammoniumoleate and this was rapidly stirred while 78.5 grams of kerosene wasadded to form an oil-in-water emulsion. In order to provide a controlsample of the film which did not contain the voids in its final form 200grams of the same casein-zinc oxidewater mixture was used containing 3grams of ammonium oleate. Both of the coatings were applied to a blankpaper so that there was a 4 mil wet coating on each of them. The sampleswere dried by heating in an air oven maintained at 260 F. for about 2minutes. The coating applied as an oil-in-water emulsion under thweconditions dried to form an essentially uncollapsed continuous filmhaving multitudinous air-casein interfaces distributed throughout. Indrying, a portion of the 'water was first removed through volatilizationtransforming the casein into a gel-like continuous film matrix havingthe kerosene droplets suspended in it in essentially the same relationship and size as they existed in the emulsion film coating, Furtherdrying volatilized the kerosene leaving the air-casein interfaces wherethe kerosene was removed. Coatings made in this manner are extremelybright and opaque.

The two coated paper samples were conditioned at 50% relative humidityfor 24 hours, and then they were charged as by the procedure describedin accordance with the description of FIG. 4 using a corona dischargemaintained at 5000 volts. Each of the samples was then exposed to lightusing a standard step tablet designed for evaluating sensitivity of amaterial to decreasing intensities of light. The step tablet used had 21steps with each succeeding step being about 0.15 density value darker.Diffuse density is a measure of the amount of incident light transmittedthrough the step wedge. It is a log function expressed as log 1/ T whereT is light transmission expressed as a decimal.

The step tablet was placed in turn over each of the charged papersamples which had been prepared as described above. Each sample was thenexposed through the step tablet to light from an Omega Enlarger (Type B6with a 75-watt PH-l 1A lamp) for 15 seconds. The samples were then tonedto produce a visible' image of the step tablet. The sensitivity(efficiency) of photoconductive films is rated by determining the firststep of lighter density than the undischarged portion of the sample;this step indicates the minimum amount of light to which the sample issensitive. In the case of the coating put on in the form of an emulsionand containing voids as required in this invention an intensitydifference was discernible between steps 6 and 7, while' in the case ofthe solid casein film coating put on as a control the intensitydifference was noted between steps 4 and 5. This is a significantdifference inasmuch as each step represents a marked improvement insensitivity.

EXAMPLE 2 This example is directed to the use of a different type ofbinder material, namely polyvinyl alcohol. Since this binder is alsosoluble in water it was made up as an oilin-water emulsion, thepolyvinyl alcohol solution being the continuous phase and forming thefilm which contained the zinc oxide photoconductive material.

50 grams of polyvinyl alcohol was dispersed in 363 grams of water byknown techniques. To this was added 300 grams of the photoconductivegrade of zinc oxide of Example 1. This gave a mixture which was 42% byweight zinc oxide. 3.5 grams of ammonium oleate was added to 200 gramsof the master polyvinyl alcohol-zinc oxide dispersion, and then withrapid stirring 84 grams of No. 9 solvent (a hydrocarbon having aninitial boiling point of 335 F. and a boiling range from 335 to 515 F.)was added to form an oil-in-water emulsion. As a control, 3.5 grams ofammonium oleate was added to 200 grams of the master polyvinylalcohol-zinc oxide dispersion. The two coatings were then applied topaper as 4 mil wet thickness coatings. Drying was accomplished as inExample 1 and the samples were conditioned at 50% relative humidity. Thecoated samples were charged as in Example' 1 and then exposed to thestep tablet as described. In the coating which contained the voids andwhich was formed from the emulsion, the intensity diference was foundbetween steps 5 and 6, while in the control the intensity differenceoccurred between steps 3 and 4.

EXAMPLE 3 This example is directed to the preparation of an emulsion ofthe water-in-oil type'. The binder used was ethyl cellulose which issoluble in benzene but insoluble in water. The master coating was formedby dissolving 41 grams of ethyl cellulose in 650 cc. of benzene. To thiswas added 1.9 grams of Triton X-400 (an emulsifier formed of stearyldimethyl benzyl ammonium chloride and related cationics) and 206 gramsof zinc oxide. The mixture was thoroughly stirred until all of the zincoxide had been dispersed. To 200 grams of the master ethylcellulose-zinc oxide mixture was added 55 grams of water with rapidstirring. This formed a Water-in-oil emulsion. The emulsion was thencoated as a film on paper and a control coating film was formed bycoating another sam- This example is directed to the use of a dye tobroaden the spectral response of a photoconductive coating containingair voids. Fit ty grams of a 12% casein dispersion was mixed with 108grams of zinc oxide (photoconductive grade as used in Example 1), 33.4grams of water, and 0.22 gram of Uranine (CI Acid Yellow 73). Anoilin-water emulsion, using casein dispersed in water as the continuousphase, was prepared by stirring 36 grams of #9 solvent into a mixture of50 grams of a 12% casein dispersion, 1.4 grams of oleic acid, 2.2 gramsof a 28% solution of ammonia in water, and 10.4 grams of water. Thisemulsion was then added to 95.7 grams of the casein-zinc oxide-Uraninemixture prepared as above. To provide a control sample which did notcontain air voids, 50 grams of a 12% casein dispersion was added to 95.7grams of the casein-zinc oxide-Uranine mixture prepared as above. Thetwo coatings were then applied to paper as 4 mil wet thickness coatings.Drying was accomplished as in Example 1, and the samples wereconditioned at 50% relative humidity. The coated samples were charged asin Example 1 and then exposed to the step tablet as described. In thecoating which contained the voids and which was formed from theemulsion, the intensity difference was noted between steps 6 and 7;while in the control the intensity difierence occurred between steps 3and 4.

EXAMPLE 5 This example is directed to showing the eifect of the size ofthe air voids on the photoconductive properties of coating containingair voids. An oil-in-water emulsion, using casein dispersed in water asthe continuous phase, was prepared by stirring 1200 grams of #9 solventinto a mixture of 1660 grams of a 12% casein dispersion, 48 grams ofoleic acid, 72 grams of a 28% solution of ammonia in water, and 350grams of water. -gram samples of the above emulsion were passed througha Manton-Gaulin Homogenizer at pressure settings of 1000 p.s.i., 2500p.s.i., and 4000 p.s.i. Particle size determinations were made on eachof the samples with the following results:

Microns Unhomogenized 2-4 Homogenized at 1000 p.s.i 0.5-2 Homogenized at2500 p.s.i 0.5-1 Homogenized at 4000 p.s.i 0.5

100 grams of a 12% casein dispersion was mixed with 216 grams of zincoxide (photoconductive grade of Example 1) and 66.8 grams of water. 95.7grams of this mixture was added to each of the 100-gram samples of theabove emulsion to give four coating formulations each having a difierentrange of particle size of the dispersed phase. Each of the four coatingswas applied to paper as a 4 mil wet thickness coating. Drying wasaccomplished as in Example 1 and the samples were con ditioned at 50%relative humidity. The coated samples were charged as in Example 1 andthen exposed to the step tablet as described. In the coating whichcontained air voids in the range of 3-4 microns an intensity diiferencewas noted between steps and 6; in the coating which contained air voidsin the range of 0.52 microns an intensity difference was noted betweensteps 3 and 4; in the coating which contained air voids in the range of0.5-1 micron an intensity difference was noted between steps 2 and 3;and in the coating which contained air voids in a range less than 0.5micron an intensity difference was noted between steps 1 and 2.

EXAMPLE 6 This example demonstrates the use of an organic filmformingphotoconductor. Since the photoconductor poly(N-vinyl carbazole) issoluble in benzene, the coating was prepared as a water-in-oil emulsion,the poly(N-vinyl carbazole) solution being the continuous phase.

To 50 grams of a 20% dispersion of poly(N-vinyl carbazole) in benzenewas added 58 grams of benzene, 1.3 grams of Triton X400 (steryl dimethylbenzyl ammonium chloride), and 0.7 gram of Arlacel C (sorbitansesquioleate). This mixture was stirred rapidly while 30 grams of waterwas added dropwise to form the waterin-oil emulsion. As a controlsample, 58 grams of benzene, 1.3 grams of Triton X400, and 0.7 gram ofArlacel C were added to 50 grams of a 20% dispersion of poly(N- vinylcarbazole) in benzene. The two coatings were then applied to paper by a#36 wire-wound rod. Drying was accomplished at room temperature for 10minutes, or until the coatings had sufficient gel strength so that theemulsion would not break. The samples were then further dried by heatingin an air-circulating oven at 260 F. for about two minutes. The sampleswere charged as in Example 1 and exposed to an ultraviolet lamp (peakwavelength 2660 A.) through a step tablet for seconds. The coating whichcontained the air voids was completely discharged by about 6% of thetotal incident radiation, while the coating without the air voids wasnot completely discharged by 100% of the incident radiation. Anintensity difference was noted between steps 10 and 11 for the coatingcontaining air voids; while in the control, which was a solid film theintensity difference was noted between steps 5 and 6.

Although it is not known precisely why the incorporation of voids into aphotoconductive film brings about the marked and unexpected increase inSensitivity shown in the examples, the following is offered as apossible explanation.

The binder together with the dark adapted photoconductive material has adielectric constant which may be denoted by EE The air in the voids inthe film has a dielectric constant E which is materially lower than thatof the coating material. The maximum voltage gradient attainable at thesurface of the coating is probably characteristic of the coatingmaterial and independent of whether or not the coating contains voids.It might be expected then that maximum voltage to which a surface couldbe charged would not be affected by bubble content in the coating (butwould depend on the coating thickness which we assume to be heldconstant). However, it appears that the introduction of voids in thecoating reduces the dielectric constant of the coating with the resultthat a lesser surface charge is required to attain this maximum surfacepotential. The discharge of the bubbled coating apparently thus involvesa lesser fiow of charge and the surface is correspondingly moresensitive to light, or is faster in the photographic sense. This isconsistent, in a qualitative way at least, with the experimentalfindings.

The decrease in dielectric constant will not, however, be directlyproportional to the voids introduced because of the tendency for theelectrostatic lines to concentrate in the region of higher dielectricconstant. On the other hand, the volume charge is probably directlyproportional to the density of the coating layer and is thus reduced indirect proportion to the voids introduced. Thus it may be inferred thatthe maximum attainable surface charge will be reduced by a lesser factorthan will the volume charge density. The result of this is an extensionof the range of linear response for a bubble-coated sheet beyond thatfound for bubble-free coatings. In this possible mechanism which ifoffered the theoretical implications of possible charge accumulations atthe free surfaces between bubbles and coating material has not beenconsidered.

There are, however, limits to the void sizes which bring about theseimproved performance characteristics. Thus any substantial numbers ofvoids larger than about ten microns are not desirable since apparentlythe presence of voids of this size contributes little to improving theperformance of the resulting photoconductive film. Data in Example 5show that any substantial number of voids less than 0.5 micron is alsoto be avoided, thus giving a preferred void size range from betweenabout 0.5 and 10 microns. In addition to being characterized in terms ofvoid size range, the photoconductive film of this invention may becharacterized by its density which should range between about 0.10 and0.6 gm./cc. without the particulate photoconductive material; e.g.,between about 0.3 and 2.5 -gm./cc. with ZnO photoconductive material.Thus density in fact limits the number of voids, or conversely, theamount of binder present, in any unit volume of film and is to someextent dependent upon the dielectric constant of the binder materialused, whether it serves in the role as a means for supporting thephotoconductive particles or as the photoconductive material itself.

The void size may be controlled in several ways. In the preferred methodof forming the photoconductive film of this invention by the use of anemulsion coating, void size is controlled by the size of the liquidglobules forming the discontinuous phase. Many techniques are known inthe art for this type of control, e.g., the use of a homogenizer (suchas a Manton-Gaulin Homogenizer).

The thickness of the finally dried photoconductive film may vary betweenabout 0.1 and 2 mils. The actual thickness will depend upon the densityof the coating and upon practical limitations associated with theapparatus used to coat the liquid on the substrates and the end use ofthe product.

The binder materials used to suspend photoconductive particles may beeither of a thermosetting nature (e.g., casein or alpha protein) or ofthe thermoplastic type. Among the thermoplastics may be listed the wellknown film forming materials such as polyvinyls, polyacetates and theircopolymers, polyolefins (e.g., polyethylene), polymers of the acrylicacids and their various derivatives, polystyrene, and elastomersincluding natural and synthetic rubbers. Mixtures of binders, casein andhydrolyzed starch for example, are also, of course, suitable. It will beseen that the film of this invention makes it possible to use a muchgreater variety of binder materials than heretofore possible.

Many of the inorganic photoconductive materials have been identifiedabove. In general they may be defined as the oxides, sulfides,selenides, tellurides, and iodides of cadmium, mercury, antimony,bismuth, thallium, indium, molybdenum, .aluminum, lead and zinc. Inaddition, mixtures of these as well as arsenic trisulfide, cadmiumarsenide, lead chromate and selenium should be mentioned. Thephotoconductive particles preferably have a high resistivity when darkadapted. The spectral response of the film will depend upon the choiceof photoconductive par ticles used; and as noted above any dyes usedwill be chosen to complement the spectral response.

These photoconductive materials are generally finely divided solidparticles which tend to agglomerate when mixed into the liquid coatingcombination. This agglomeration, which is apparently desirable, meansthat ultimate particle size is relatively unimportant. Generally, thesensitivity of the coating decreases as the size of the photoconductoragglomerates decreases. In formulating the coating it is desirable todisperse the photoconductive particle agglomerates only enough to give asmooth coating on the substrate.

Because the photoconductive particles in the film described herein aresuspended in a binder also containing voids some consideration must begiven to the weight ratio of photoconductive particles to bindermaterial. The minimum amount of photoconductive particles is largelydetermined by the performance desired from the photoconductive film andcan generally be defined as 4 parts by weight to one part of binder. Theupper limit is primarily determined by the quantity of photoconductiveparticles which can be held within the binder and it may generally bedefined as about 8 to 10 parts by weight to one part of binder.

The substrate may be either an insulating material such as paper, or anelectrically conducting material such as metal foil or paper loaded withcarbon black. It preferably has a lower resistivity than the filmattached thereto. The substrated may be flexible, semi-flexible orrigid, depending on its ultimate use. For ofiice copying it willnormally be a thin paper.

It will be seen from the above examples that the voidcontainingphotosensitive coating of this invention is considerably more responsiveto light exposure than an equal weight of the same coating withoutvoids. This means that using lighter coating weights the coatingdescribed herein achieves a response equivalent to those now in use.These lighter coating weights in turn offer the possibility of usinglighter-weight supporting substrates, a fact which contributes to theoverall decrease in weight. Moreover, the coating of this inventionoffers the possibilty of using a wider range of binders than heretoforepossible and of forming photoconductive coatings which are pressuresensitive and moisture insensitive.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding descrip tion, are efficiently attained and,since certain changes may be made in carrying out the above process, inthe described product and in the constructions set forth withoutdeparting from the scope of the invention, it is in tended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

I claim:

1. A printing base suitable for electrophotographic reproductioncomprising, in combination:

(a) a substrate; and

(b) an essentially continuous photoconductive film carried thereon, saidphotoconductive film containing voids throughout, substantially all ofsaid voids being in the size range of 2 to 10 microns, whereby thephotoconductive sensitivity of the film is materially increased.

2. Printing base in accordance with claim 1 wherein said photoconductivefilm comprises a binder and solid photoconductive particulate materialuniformly distributed throughout said binder.

3. Printing base in accordance with claim 2 further characterized inthat said photoconductive film contains a dye capable of widening thespectral band to which said photoconductive particles are responsive.

4. Printing base in accordance with claim 1 wherein said photoconductivefilm is an organic film-forming photoconductive material.

5. Printing base in accordance with claim 4 wherein said organicfilm-forming photoconductive material is poly(N-vinyl carbazole).

6. Printing base in accordance with claim 1 wherein said substrate is adielectric material.

7. Printing base in accordance with claim 1 wherein said substrate is anelectrically conducting .foil.

8. Printing base in accordance with claim 2 wherein said binder materialis casein.

9. Printing base in accordance with claim 2 wherein said photoconductiveparticulate material is zinc oxide.

10. A method of copying electrophotographically, comprising the stepsof:

(a) applying to a backing sheet an essentially continuousphotoconductive film containing throughout air ranging in size betweenabout 2 and 10 microns;

(b) forming an electrostatic image on the surface of said film;

(c) toning said image; and

(d) fixing the resulting toned image whereby it becomes a permanentvisible mark on said sheet.

11. A method in accordance with claim 10 wherein said photoconductivefilm comprises a nonphotoconductive binder having solid photoconductiveparticulate material throughout said continuous film.

12. A method of copying electrophotographically, comprising the stepsof:

(a) applying to a backing sheet an essentially continuousphotoconductive film containing throughout voids ranging in size betweenabout 2 and 10 microns;

(b) forming an electrostatic image on the surface of said film;

(c) toning said image;

(d) transferring the resulting toned image to a substrate; and

(e) fixing the transferred toned image whereby it becomes a permanentvisible mark on said substrate.

References Cited UNITED STATES PATENTS 2,857,271 10/1958 Sugarman 961.82,955,938 10/1960 Steinhilper 961.5 3,108,009 10/1963 Clancy et a]11736.7 X 3,121,006 2/1964 Middleton et al 96-1.5 3,255,039 6/1966Dalton 1l736.7 X 2,297,691 10/ 1942 Carlson 961 2,663,636 12/1953Middleton 96l 3,037,861 6/1962 Hoegl et al 96-1 3,234,017 2/1966 Heyl eta1 96-1 NORMAN G. TORCHIN, Primary Examiner.

C. E. VAN HORN, Assistant Examiner.

